The biological bases of hunger and thirst needs are explained through various internal physiological mechanisms and external factors that regulate these essential drives.

Hunger is triggered by multiple internal stimuli. These include stomach contractions that signify the stomach is empty, a low concentration of glucose in the blood, a low level of protein, and the amount of fats stored in the body. The liver also plays a crucial role by responding to the lack of bodily fuel and sending nerve impulses to the brain. External factors such as the aroma, taste, appearance of food, observing others eating, and the smell of food also create a desire to eat. Our food intake is regulated by a complex feeding-satiety system located in the hypothalamus, liver, and other body parts, along with external environmental cues.

Thirst is controlled by processes within the body. When deprived of water for several hours, the mouth and throat become dry, leading to dehydration of body tissues. The motivation to drink water is triggered by loss of water from cells and reduction of blood volume. When water is lost from bodily fluids, water leaves the interior of the cells. The anterior hypothalamus contains nerve cells called 'osmoreceptors', which generate nerve impulses in case of cell dehydration. Water must sufficiently enter the tissues to remove the dryness of the mouth and throat.